High frequency (HF) communication systems for example, exhibit multipath signal delay spread characteristics that can cause intersymbol interference (ISI) and, hence, multipath fading at a receiver thereof, resulting in a severe degradation in the performance of such communication systems. Several techniques have been employed to combat such multipath fading, namely, the use of parallel tone transmission techniques, the use of serial tone transmission techniques, or the use of adaptive equalization techniques.
In the parallel tone technique, the modem simultaneously sends a number of waveforms at different subcarrier frequencies in order to provide longer symbols and thereby make the delay spread only a small fraction of the symbol duration in order to reduce ISI. Typical of this technique is the so-called "Kathryn" modem, developed by General Atronics Corporation and described in the article, "The AN/GSC-10 (Kathryn) Variable Rate Data Modem for HF Radio", IEEE Transactions on Communicaton Technology, Volume COM-15, No. 2, April 1967, pp. 197-204. The Kathryn modem is relatively simple but has difficulty in combatting multipath fading and, further, is sensitive to frequency error due to the long symbol duration. Such parallel tone modems also suffer from the effects of peak-to-average signal loss since the radio transmitter must operate with a sufficient back-off of the transmitter amplifier to accommodate the voltage variations in the sum of the subcarriers.
Examples of adaptive equalization techniques are discussed in the article "Feedback Equalization for Fading Dispersive Channels", by P. Monsen, IEEE Trans. on Information Theory, Vol. IT-17, No. 1, January 1971, pp. 56-64, and in U.S. Pat. Nos. 3,879,664 and 4,328,585 issued to P. Monsen on Apr. 22, 1975 and May 4, 1982, respectively. In such an approach, the modem employs adaptive techniques to track the amplitude and phase of each multipath return signal to combine the energies in the various paths (e.g., by matched filtering) and/or to remove ISI (e.g., an equalization technique). One or more transversal, or tapped delay line (TDL), filters are usually employed for these functions. This type of modem is quite complex in its configuration and operation and, while it works well under slow fading conditions, it tends to have difficulties in tracking under fast fading conditions. The complexity and the tracking difficulty are due to the fact that both the phase and amplitude of the tap weights are adapted and that the number of taps required is relatively large.
A serial tone technique is discussed in the article "HF ACARS.sup.SM Signal In Space Specification," published by Aeronautical Radio, Inc. (ARINC), ARINC Document No. SE-84012, dated Mar. 12, 1984. In the system of the type discussed in the article, the modem is designed for data transmission at 300 bits per second (bps) and uses error-correction encoding to reduce the signal-to-noise ratio required to support reliable operation. The modelm transmits a preamble signal and encoded information bits, each bit being band spread by applying a seven bit Barker sequence to each bit. The receiver uses the preamble signal to acquire the frequency of the transmitted signal, the MSK chip period (where the transmitter uses MSK modulation) and the transmitter bit symbol perod and then demodulates the encoded signal and decodes the demodulated signal. Such an approach is not as effective as desired since the use of the specified Barker sequence does not appear to provide a good band spreading characteristic. Further the format of the encoded information proposed therein does not appear to be very effective. As a result, the modem does not respond to enough multipath signals to provide the desired operation, does not provide adequate resolution over the desired range of Doppler shifts, and tends to produce undesirable sidelobe properties in the modem signal.
It is desirable, therefore, to overcome the disadvantages of the above systems and to provide an improved serial tone modem using non-equalization techniques.